Light sources such as light-emitting diodes (LEDs) are an attractive alternative to incandescent and fluorescent light bulbs in illumination devices due to their higher efficiency, smaller form factor, longer lifetime, and enhanced mechanical robustness. However, the high cost of LEDs and associated heat-sinking and thermal-management systems have limited the widespread utilization of LEDs, particularly in broad-area general lighting applications.
The high cost of LED-based lighting systems has several contributors. LEDs are typically encased in a package, and multiple packaged LEDs are used in each lighting system to achieve the desired light intensity. In order to reduce costs, LED manufacturers have developed high-power LEDs that emit relatively higher light intensities by operating at higher currents. While reducing the package count, these LEDs require higher-cost packages to accommodate the higher current levels and to manage the significantly higher resulting heat levels. The higher heat loads and currents, in turn, typically require more expensive thermal-management and heat-sinking systems which also add to the cost (as well as to the size) of the system. Higher operating temperatures may also lead to shorter lifetimes and reduced reliability. Finally, LED efficacy typically decreases with increasing drive current, so operation of LEDs at higher currents generally results in a reduction in efficacy when compared to lower-current operation.
A further problem associated with using fewer high-power LEDs in broad-area lighting—for example, to replace fluorescent lighting systems—is that the light must be expanded from the relatively small area of the die (on the order of 1 mm2) to emit over a relatively large area (on the order of 1 ft2 or larger). Such expansion often results in decreased efficiency, reduced performance, and increased cost. For example, a light panel may be edge-lit and incorporate features that redirect or scatter light. However, it is often difficult to achieve uniform light intensity over the entire emitting area of such panels, with the intensity generally being higher at the edge(s) near the light sources. Also, the emission pattern from such devices is typically Lambertian, resulting in poor utilization of light and relatively high glare.
An alternate approach to producing broad-area lighting is to use a large array of small LEDs positioned over the desired emitting area. Such LEDs may be unpackaged LEDs (i.e., LED dies) or packaged within, e.g., a leadframe and polymeric encapsulation. This tends to reduce the cost and efficiency losses associated with optics required to spread out light from a small number of high-power LEDs. However, this approach typically involves forming an array of a very large number of light emitters over a relatively large area. In many such approaches the substrate upon which the light emitters are formed may be mated with other materials to aid in integration of phosphors, optics or for protection of the light-emitter sheet.
Lighting systems have a wide range of specifications, for example for luminous efficacy, light output power, color temperature, color rendering index (CRI) and the like. Many of these specifications are related to the LEDs, the light-conversion material (utilized to shift the wavelength of light from the LEDs to another wavelength, resulting in, e.g., white light), and the interaction between the two. In particular, various specifications in large measure determine the luminous efficacy, the color temperature, and the CRI. Uniformity of these characteristics is another key specification for lighting systems and the uniformity of the luminous efficacy, color temperature, and CRI are typically directly dependent on the homogeneity of the LED and light-conversion materials.
In array based lighting systems it can be difficult to achieve acceptable uniformity of the light-conversion material. Typically the phosphor powder is mixed in a binder, for example silicone, and this is applied or dispensed to the LEDs. The phosphor powder may segregate in the binder, resulting in a non-homogeneous distribution of phosphor in the binder associated with each LED. A second complication is that the binder may start to cure, even at room temperature, during the application process. Partial curing of the binder during the application process may result in non-uniform phosphor coverage.
These issues may apply to many types of phosphor-converted light emitters, including single-LED packaged devices, multiple-LED packaged devices, arrays of LED and single or arrays of unpackaged LED (die) to which phosphor is applied. In some systems it is desirable to integrate the LEDs and phosphor with one or more optical elements (e.g., a lens) to control the light-distribution pattern. Optical alignment of the LED and phosphor with the optical element(s) is often important to achieve the desired light-distribution pattern and high optical efficiency.
In view of the foregoing, a need exists for the uniform and low-cost application of phosphors to LEDs, and in particular either selectively or with full coverage over arrays of LEDs, as well as for economical, reliable LED-based lighting systems based thereon. A need also exists for improvements in integration and alignment of optical elements with LEDs and phosphors.